The present invention relates to a process which makes it possible to control the viscosity or the average molecular mass of a polyisobutene manufactured continuously in a polymerisation reactor.
It is known to polymerise isobutene continuously in a reactor comprising a boiling liquid reaction phase containing the monomer and the forming polymer, above which liquid phase is a gas phase comprising especially the monomer in equilibrium with the liquid phase. The continuous polymerisation is especially carried out by continuously introducing the monomer and a catalyst into the reactor and by continuously withdrawing from the reactor the liquid phase which is then generally subjected to one or more purification steps intended to isolate the polyisobutene manufactured.
The monomer often consists of isobutene coming from a mixture of butenes and/or of butanes.
In general, the polymerisation reaction is carried out continuously with the aid of a cationic-type catalyst and optionally a cocatalyst.
In a continuous polymerisation, the monomer feed, that is to say the isobutene feed, is generally produced from a cut of essentially C4 hydrocarbons, i.e. from a mixture containing isobutene, from other C4 olefins and/or from C3 to C7, especially C4, alkanes. The quality of the monomer feed may vary over time so that it affects the polymerisation conditions and consequently the quality of the polymer obtained.
Similarly, the quality of the catalyst and/or of the cocatalyst may vary over time and also may affect the quality of the polymer produced, particularly when the catalyst activity is very sensitive to poison variations.
The applications of polyisobutenes are often tied to their rheological properties. One of the essential characteristics of polyisobutene is its viscosity or its average molecular mass.
In a process for the continuous manufacture of polyisobutene, the average residence time of the polymer in the polymerisation reactor may be relatively long. Furthermore, the reaction mixture continuously withdrawn from the polymerisation reactor undergoes one or more polymer purification steps. The final polymer is thus isolated and purified after an additional time which may generally be several hours, for example from 3 to 12 hours, so that any analysis of the polymer after this last step is carried out very late. Consequently, the elapsed time between a measurable drift obtained from the analysis of the viscosity or of the average molecular mass of the polyisobutene and the correction of the said drift in the polymerisation reactor is relatively long. This type of drift therefore causes a product to be manufactured which is outside the viscosity or average molecular mass specifications, and this is generally so to a not insignificant extent.
Processes have been sought in the past to solve, in part, the abovementioned problems.
French Patent Application FR 2,625,506, discloses a process for the polymerisation of olefins, e.g. a mixture of butene monomers, wherein the number average molecular mass of samples of the polymer is determined by infrared spectroscopy. The datas are used to control the feed rate of the monomer mixture into the polymerisation reactor. A regulation using this process is also described, but this does not solve the problem of the present invention. In particular, nothing is said about measuring the isobutene partial pressure in the gas phase of the reactor.
The problem to be solved is to find a control process which makes it possible to correct the fluctuations in the viscosity or in the average molecular mass of the polyisobutene and therefore to adjust more rapidly the polymerisation conditions in the reactor so as to limit the amount of polyisobutene manufactured outside the specifications.
French Patent FR 2,790,760 (application Ser. No. 99/03267) describes a process for maintaining the viscosity or the average molecular mass of polyisobutene at a desired value in a continuous isobutene polymerisation process, wherein the partial pressure of the isobutene in the gas phase of a polymerisation reactor is held constant and wherein the partial pressure is influenced by the flow rate of the C4 hydrocarbon feed mixture and/or the flow rate of catalyst introduced into the reactor. This type of process had been employed especially because of the difficulty in measuring the isobutene concentration in a very viscous liquid polymerisation medium such as the boiling liquid reaction phase of the reactor.
French Patent FR 2,794,757 (application Setr. No. 99/07649) describes an improved process for maintaining a property P selected from the viscosity or the molecular weight of a polyisobutene at a constant desired value by correlating said property P with the partial pressure PiC4 of isobutene in the gas phase of a polymerisation reactor, calculating a corrected partial pressure for isobutene (PiC4)c from PiC4, from the polymerisation temperature and from the concentration of at least one of the components of the C4 hydrocarbon feed, and holding constant (PiC4)c at a given target value of the property P by acting on the introduction flow rates of the catalyst and of the C4 hydrocarbon feed in the reactor. The objective of this process is to be able to reduce, or even eliminate, the impact of various parameters that may modify the liquid/vapour equilibrium and influence the partial pressure PiC4 but without, as a result, directly affecting the viscosity or the average molecular mass of the polyisobutene manufactured.
Despite the improvements made by the above process, fluctuations in the viscosity or in the average molecular mass of the polyisobutene can nevertheless be seen.
U.S. Pat. No. 4,620,049 describes a process for controlling the molecular mass of a polybutene (PB). In this process, the molecular mass is correlated with operating variables selected from the polymerisation temperature and the concentration of isobutene in the reactor. Then a molecular mass of the product output is calculated and depending on the desired molecular mass, the polymerisation temperature and/or the concentration of isobutene can be altered. Moreover, moisture can be added to the reactor feed for lowering the molecular mass of the PB.
Nothing in these processes is said about measuring the isobutene partial pressure PiC4 in the gas phase of the reactor, about calculating a predicted value of the isobutene concentration in the boiling liquid reaction phase of the reactor with the help of a first model and from the measured value of PiC4, and about calculating a predicted value of the property P with the help of a second model comprising a function of the isobutene polymerisation kinetics and from the said predicted value of the isobutene concentration. However, according to the present invention it was observed that these missing elements are considered to be essential in order to obtain a more refined regulation of the property P.